v5.2: stability and polish — deep-audit bug fixes + install-flow fixes

Verified findings from a multi-dimension code audit, plus the two install-flow bugs:
- Fix Opus encoder use-after-free on a codec/rate change while streaming (guard swap vs encode).
- Fix "both" single-file recording dropping audio + drifting (drain both directions in lockstep).
- Fix broken clip counter, UPnP teardown on exit, auto-update-restart foreground grant, and a
  malformed-Opus-format packet orphaning a playout session forever.
- Post-install relaunch now respects start-minimised; uninstall is path-aware so it won't clear a
  different copy's run-at-startup.
- Perf/hygiene: cache AppConfig off UI hot paths, fold per-peer EQ+gain into one pass, deterministic
  disposal (tray menu, timers, COM shortcut, Process handles, process meter), ring-buffer overflow
  guard, receiver session-lock fix, remote-control allow-list moved onto the UI thread.
- Remove dead code (two IsAsioBackend, SessionPlayout.Reset, IsSameEndpoint, RemSoundUpdater
  IDisposable); several stale-doc fixes.

Deferred (not in this release): drift-estimator tweak, peer-discovery pruning, uninstall retry-loop,
encryption nonce. Wire format unchanged (interops v3.3-v5.1). Version -> 5.2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/RecordingController.cs

@@ -149,6 +149,17 @@ internal sealed class RecordingController
 
     private void StartMultiTrack(RecordingSettings s, DateTime now)
     {
+        // Refuse a split received-only recording with no peers connected: it would make an empty dated
+        // folder and capture nothing while still announcing "recording started". Fail clearly instead
+        // (Start()'s catch surfaces this message). Both / sent-only still record your own send, so they
+        // never hit this. Checked BEFORE creating the folder so no orphan folder is left behind.
+        if (s.Source == RecordingSource.ReceivedOnly && (ConnectedPeersProvider?.Invoke() ?? []).Count == 0)
+        {
+            throw new InvalidOperationException(
+                "No peers are connected, so a split received-only recording would capture nothing. " +
+                "Connect to a peer first, or set the recording source to include your own sent audio.");
+        }
+
         var folder = MultiTrackFolder(s, now);
         Directory.CreateDirectory(folder);
         var ext = AudioRecorder.ExtensionFor(s.FileFormat);